Pirh2 interacts with and ubiquitylates signal recognition particle receptor beta subunit

Pirh2 is a RING finger type ubiquitin ligase which ubiquitylates various proteins including p53, p27(Kip1), HDAC1, and epsilon-COP. In this study, we identified signal recognition particle receptor beta subunit (SRbeta), an integral membrane protein of the endoplasmic reticulum (ER), as a novel Pirh2-interacting protein by yeast two-hybrid screening. We confirmed that Pirh2 interacted with SRbeta in mammalian cells. An immunofluorescent staining revealed that Pirh2 colocalized with SRbeta in the ER. Pirh2 poly-ubiquitylated SRbeta in an intact RING finger domain-dependent manner in vivo and in vitro. Unexpectedly, different from other Pirh2 substrates, neither overexpression of Pirh2 nor depletion of cellular Pirh2 affected SRbeta protein stability. Pirh2 preferentially utilized lysine residues 6 and 29 of the ubiquitin to mediate the formation of polyubiquitin chains on SRbeta. These results suggest that Pirh2 may regulate SRbeta function by mediating poly-ubiquitylation of SRbeta without affecting the stability of SRbeta protein per se.